1. Field of the Invention
The present invention relates to a medium thickness detecting apparatus and an image forming apparatus.
2. Description of the Related Art
A conventional image-forming apparatus such as a printer, copying machine, and facsimile machine includes a photoconductive drum, charging roller, recording head, developing unit, transfer roller, and fixing unit, and forms an image through a predetermined image-forming processes.
The recording head is driven to form an electrostatic latent image on the surface of the photoconductive drum and the developing unit develops the electrostatic latent image into a toner image. The transfer roller transfers the toner image onto a recording medium and the fixing unit fixes the toner image into a permanent image. The fixing unit includes a heat roller and a backup roller, so that the heat roller heats the toner particles and the backup roller presses the heated toner particles against the recording medium.
However, when the toner is fused onto the recording medium, the amount of heat given to the toner varies depending on the heat capacity and temperature of the recording medium, resulting in so-called cold off-set and hot off-set that deteriorate print quality.
A tandem type image-forming apparatus includes a plurality of image-forming sections arranged along a path in which an endless transport belt runs, each image-forming section forms a toner image of a corresponding color. The recording medium passes through the respective image-forming sections in sequence so that the respective toner images are transferred one over the other in registration with one another other to form a color toner image. The color toner image has a thick layer of toner by its nature. The thicker toner layer increases the chance of offset occurring and causes poorer print quality.
One way of preventing off-set of the image is to detect the thickness of the recording medium and changes the conditions of image forming process of the respective image forming sections in accordance with the detected thickness of the recording medium. However, the accuracy of detecting the thickness cannot be improved easily.